The placement of electronic components using self assembly is becoming an important approach for high-volume production of electronic assemblies. For example, it is well-known to use fluidic self assembly (SA) in the production of radio-frequency identification tags (RFIDs). In that approach, sub-millimeter integrated packages with distinct dimensions and trapezoidal shapes are dropped into an agitated fluid where they fit into specific matching depressions on a substrate. Packages that don't fall into depressions are removed and redropped until all are matched. Circuit connections are then made by masking and depositing conducting strips over the electronic packages. This approach works well at high volumes, but requires very specific geometrically-shaped components or packages and substrates which have to be specially etched to accommodate the packages.
A more general approach that has been investigated does not require specially-shaped packages and can use more standard components. In this approach, components are dropped into an agitated fluid where they find proper locations on a substrate through contact and adherence using various approaches. For example, hydrophilic and hydrophobic materials may be coated on the components and desired substrate locations, or bonding sites, such that when parts find proper locations they tend to stick when the same coatings come into contact, i.e., hydrophilic-hydrophilic or hydrophobic-hydrophobic; mixed coatings do not stick. Agitating the fluid is also necessary since it randomized the motion of the components, allowing them to make contact attempts with all regions of the substrate. Furthermore, if they don't stick on the first attempt, agitation allows the components to make many attempts until they finally find a bonding site.
One of the best ways to achieve the self assembly is by using the strong wetting effects of solder on a metal contact to “pull” components into place. Unlike other SA bonding materials, solder also has a high lubrication; this implies that once the component makes contact with the solder, the component can find the minimum energy configuration with minimal friction. This wetting effect occurs when the solder is a liquid, therefore self assembly of components must be done above the melting point of the solder. In the case of solder SA, one immerses the substrate and the electronic components in a liquid, allowing the liquid to carry components into their positions. The solder wetting effect takes over when the components come into contact with the melted solder on the substrate, pulling the components into their final position and retaining them. Note that in particular, using solder fluxes as the binding agent is not useful for SA because of their low degree of lubrication.
For solder SA, low melting temperature solder (Tm<150° C., where Tm is the melting point) is used for a variety of reasons. One reason is that simple lower viscosity non-toxic fluids such as water are easy to use, but obviously require temperatures to be below their boiling point. Furthermore, since electronic or opto-electronic components are immersed in the hot liquid on the order of one minute in typical SA runs, high temperatures may damage the components. Unfortunately, very low temperature solders (Tm<100° C.), generally require Bi which generally leads to poor bonding and therefore unreliable long term attachment of components. Solder compositions such as Sn—In can have Tm=145° C., but again reliable bonding is not acceptable for long term attachment of components. In addition, use of solders with such low melting points may be problematic for long term operation of components such as light emitting diodes (LEDs) whose operating temperatures may approach or even exceed the melting point of such low temperature solders.
U.S. Patent Publication No. 2010/0139954 to Morris et al. discloses an approach by which solder or fluid based-SA can be performed at practical temperatures while still providing a method to permanently electrical bond components with reliable higher temperature solders. The approach uses multiple sites that perform different functions. In particular, a central site on the component is used for a SA binding site while spatially separated sites closer to the part boundaries are used for electrical bonding. Generally the electrical bonding sites are solder bumps. All contacts are on the bottom of the component and are designed to mate with matching sites on the substrate. The central binding site on the substrate supports a low temperature solder (or other material) that forms a hemispherical shape when liquefied. The height of the central SA solder when liquid exceeds the height of the solid electrical solder bumps. The solder bumps melt at a higher temperature than the central binding site solder. In the embodiment described, eutectic Bi—Sn solder (Tm=138° C.) is employed for SA binding sites which bind to solder bumps on electrical components. The solder bumps are composed of eutectic Sn—Pb (Tm=183° C.) which are well known to form reliable, high conductivity electrical connections. Assembly is performed in two steps. In the first step, components and substrate are placed in a fluidic bath at a temperature above the melting point of the solder or material on the central SA biding site, but below that of the solder bumps. Self-assembly onto the central pads is performed in the liquid bath. When components contact the central SA solder that is on the substrate, the bulging profile relaxes because of the additional wetting of the component contact. The assembled substrate is cooled to fix the component locations. The substrate is then placed into a reflow oven where temperatures are above the melting point of the solder bumps which must then expand to reach the contacts on the components. While this approach permits electrical connections with more reliable and higher conductivity, components and substrates require additional contacts and pads which lead to greater fabrication complexity. More significantly, practical applications of this method require components with solder bumps and additional solder masks for coating only the SA binding sites with low temperature solder. This leads to longer overall manufacturing times and cost, both of which SA should alleviate. Other problems occur with this method because the physical height changes of the SA solders and electrical bonding solders must be compatible with the process.